a. Polymer Foam Technology
Open-cell foams have many applications, including bioscaffolds[1-3], high surface area catalysts[4, 5], filters[6], battery-capacitors and fuel cell electrodes [7, 8], vibration/acoustic dampers[9, 10], thermal insulation[11, 12], and sorbents[13, 14]. Polymeric open-cell foams are of particular interest due to their relatively low cost, low weight and wide range of physical and mechanical properties.
There are four basic mechanisms of manufacturing polymer foams: gaseous blowing agents, liquid blowing agents, emulsion templating and porogen (a solid particulate pore former) leaching. These mechanisms can produce both open and closed cell foams, with the transition between the two structures relying upon the percentage avoid space created in the particular foam matrix [15]. Each of the basic mechanisms can be broken down into a variety of specific methods. For example, a gaseous blowing agent can be physically blown/injected into a viscous polymer and then heated to expand into pores, or gas molecules could be a byproduct of the polymerization reaction and collected into bubbles that become the cells of the foam[16]. A blowing agent that does not undergo a chemical reaction during the foaming process is termed a physical blowing agent, whereas chemical blowing agents create gases from a chemical reaction. Of course, each of these methods has its advantages and drawbacks. Gas bubbles (or liquid droplets) generated by gaseous blowing agents, liquid blowing agents and emulsion templates need to be stabilized (usually with a surfactant) to prevent their coalescence so that large voids do not occur in the final polymer foam. The bubbles/droplets in these known methods can also be stabilized by low surface energy particles at the bubble (or droplet) interface or by both particles and surfactants. The latter means of stabilization results in the formation of a hierarchical pore structure, i.e., an array of pore sizes of different length scales, i.e., a variety of micropores and macropores. The small pores are due to surfactant stabilized droplets and larger pores are due to particle stabilized droplets[17]. A hierarchical pore structure gives foams improved absorption capabilities over uniform cell size foams due to the differing transport rates of fluids through large and small pores in the foam's structure[18], as well as improved mechanical properties[19].
Porogen leaching methods generally do not have to be stabilized, but the solid porogen must be removed from the polymer matrix, which can be a cumbersome and wasteful process, especially as the size of the porogen (and hence pores of the foam) is reduced. However, templating and subsequent leaching of solid fillers is one of the simplest and most robust techniques for creating open-cell structures. Common solid porogens include sugar (sucrose)[20-30] and salt (sodium chloride)[23, 28-40] because they are easily dissolved in water, as opposed to polymeric porogens or fillers that require the use of organic solvents for dissolution. Ice has also been shown to be an effective solid porogen[41-43], as it can easily be melted away.
There are other more complex methods of creating polymer foams, such as using micelles to create the pores of the foam [44, 45] or by stabilizing aqueous foams solely by polymer particles at the liquid-gas interface and then sintering the polymer particles into one solid structure [46-49].
Just as there are many methods of creating polymer foams, there are also many different types of polymers from which foams can be created. Foams having a wide range of properties may be made from thermoplastic and thermoset polymers, biopolymers, hydrophobic and hydrophilic polymers and elastomers. Commonly foamed polymers include polystyrene (especially for consumer food containers), polyurethane, polyvinylchloride, polypropylene, polyethylene and silicone.
b. Oil Spill Remediation Technology
The release of petroleum hydrocarbons into a marine or other aqueous environment, commonly known as an oil spill, has tremendous negative environmental and economic impacts [50-51] and are extremely costly and time consuming to clean up. Environmental repercussions of oil spills include the loss of animal life and habitats, and the introduction of harmful chemicals into their food supply [52-57]. Aside from the obvious economic effects of reduced profits to oil companies and increased gasoline prices [58], oil spills can destroy the main source of income for fishing communities and create economic downturns in travel destinations [59]. In the case of the British Petroleum Gulf of Mexico oil spill in 2010 over $20 billion has been set aside for litigation and environmental remediation costs [60].
The catastrophic nature of oil spills has provided impetus for the development of remediation techniques, including various methods for effectively removing oil from water. There exist large and expensive equipment that use oleophilic (likes oil)/hydrophobic (hates water) spinning disks to selectively adsorb oil to their surface as a protrusion scrapes the oil off. However, these machines are only effective close to the site of the spill where the concentration of oil is large [61, 62]. A centrifuge that spins a mixture of oil and water can also be used to separate oil from water based on their different densities, as the oil will remain in the center of the centrifuge while the water is pushed to the edges [61]. These machines can process large volumes of contaminated water; however the concentration of oil in the processed water would still be above the environmentally safe level of 10 ppm [57, 63]. Skimmers, as their name implies, attempt to skim the oil slick off of the surface of the water, although they require calm water to be effective[64]. These techniques are used where there is a high concentration of oil, usually near the site of the spill, but are not effective as the oil slick spreads far away [65-67]. Dispersants are used to break up an oil slick into oil droplets that can be diluted into the volume of the water rather than coalesce at the surface. Dispersants themselves tend to be toxic, especially in conjunction with oil [57, 68].
Further away from the site of the spill, absorbent materials can be used as they can more selectively absorb oil that exists in lower concentrations [69]. Though oil and water are immiscible, most porous, spongy materials will readily absorb both oil and water [70, 71]. This is undesirable because the oil tends to create a thin film on the surface of the water that is easily emulsified (albeit unstably), with the result that water is just as likely to be absorbed as the oil. This makes non-selective absorbent materials uneconomical for oil cleanups.
Silicone open-cell foams are of particular interest because they have been shown to be able to separate oil from water due to the inherent hydrophobicity and oleophilicity of silicone[22]. This property coupled with a high chemical resistance, a low glass transition temperature (−125° C.) and stability at high temperatures[72] make silicone foams ideal for cleaning up oil spills in harsh environments. Not only are the physical properties of the silicone foam important, but so is the morphology of the cellular structure. Recently there has been much interest in creating hierarchical pore morphologies in foams[73-82] due to their improved mechanical and physical properties.